Process for producing metal powders and the like



United States Patent 3,399,051 PROCESS FOR PRODUCING METAL POWDERS AND THE LIKE John F. Hardy, Andover, and Merrill E. Jordan, Walpole, Mass, assignors to Cabot Corporation, Boston, Mass., a corporation of Delaware N0 Drawing. Continuation-impart of application Ser. No. 513,545, Dec. 13, 1965. This application Dec. 15, 1966, Ser. No. 601,861

7 Claims. (Cl. 75-.5)

ABSTRACT OF THE DISCLOSURE This invention relates to improvements in the process for producing metals by heating to reducing temperatures a mixture comprising a reducible metal compound and carbon under non-oxidizing conditions; said improvement comprising producing said mixture by providing a solution comprising a soluble, reducible metal salt or complex salt and an inert solvent, mixing into said solution particulate carbon and removing the solvent from the resulting mixture.

This invention relates generally to a process for producing metal powders and more particularly to a process for producing finely-divided metal powders under unusually mild conditions. This application is a continuation-in-part of copending US. patent application Ser. No. 513,545, filed Dec. 13, 1965, now abandoned which in turn is a continuation-impart of US. patent application Ser. No. 387,223 filed Aug. 3, 1964, now abandoned.

It has long been known that certain metal compounds can be reduced to the corresponding metal when heated under non-oxidizing conditions in the presence of carbon. Said type of process has been utilized successfully for the production of various metals such as nickel, cobalt, iron, tungsten, and the like.

In our aforementioned applications there are disclosed advantageous processes for producing finely-divided metal powders which broadly comprise (1) forming a mixture comprising (a) a liquid medium, (b) certain carbons, and (c) a reducible metal compound; (2) conducting the resulting mixture through a heated drying zone, thereby removing said medium; and (3) heating the resulting dried metal compound/ carbon composition to a temperature sufficient to convert said metal compound .to the corresponding metal. Preferably, the drying step is effected by spray drying of the mixture. Said processes provide efficient, versatile and relatively simple and inexpensive methods for producing finely-divided metal powders.

In accordance with the present invention it has been further discovered that certain beneficial and completely unexpected advantages acrue to processes of the aforementioned type when the reducible metal compound is utilized in solution to form the metal compound/carbon composition.

Accordingly, it is a principal object of the present invention to provide a novel improvement in a process for the production of metals.

It is another object of the present invention to provide improved unusually readily reducible metallurgical compositions.

Other objects and advantages of the invention will in part be obvious and will in part appear hereinafter.

In accordance with the present invention it has been discovered that unusually easily reducible metal compound/carbon compositions result when certain particulate carbons are dispersed in a solution comprising a reducible metal compound and the solvent thereafter removed.

Patented Aug. 27, 1968 Broadly, the metal compounds forming part of the reducible metallurgical compositions of our invention include those metal compounds which can be converted to the corresponding metal by carbon. Thus, the metal compounds of interest include compounds of copper, tungsten, zinc, lead, tin, iron, cobalt, nickel, manganese, chromium, vanadium, molybdenum, and mixtures of these. Especially preferred are the water soluble organic and inorganic salts and complex salts of the above-mentioned metals. Representative preferred compounds include the sulfates, nitrates, chlorides, bromides, iodides, fluorides, perchlorates, orthoarsenates, sulfides, acetates, citrates, oxalates, formates, benzoates, carbonates, oleates, and tartrates of the above-mentioned metals. The benefits which flow from the practice of our invention are especially apparent when the salts utilized are those which can be converted to the free metal (in the absence of carbon) at temperatures above about 500 F. but below about 3000 F.

The particulate carbon utilized in the reducible metallurgical compositions of the present invention are characterized by 1) a surface area of at least about 5 m. gram and (2) an equilibrium moisture adsorption factor of at least about 0.25%.

By surface area it is meant the total exposed (internal and external) surface of the carbon. Thus, the method by which said surface area is determined is important. For the purposes of the present specification and claims all surface areas are determined by the extent of nitrogen adsorption on the carbon according to the Brunauer- Emmett-Teller method. Generally speaking, carbons having a BET N surface area of greater than about 25 m. gram are greatly preferred.

The surface of carbon is complex and usually comprises, in addition to carbon, certain other foreign atoms such as oxygen and/or hydrogen atoms. The determination of the quantity and quality of said atoms normally involves complex and time consuming chromatographic or spectrophotometric procedures. However, an empirical value can be placed upon such surface atoms by measurement of the moisture adsorption factor of the carbon. For the purposes of the present invention, carbons having a moisture adsorption factor of greater than about 2% are much preferred. Generally speaking, the greater said moisture adsorption factor the greater the number of foreign atoms (i.e. other than carbon) on the surface of the carbon. Determination of said moisture adsorption factor is achieved, for the purposes of the present specification and claims, by (1) drying the carbon sample at about 105 C. for about 16 hours, (2) exposing the dried sample to moisture in a humidity chamber maintained at about 22 C. and about relative humidity for a period of time sufficient to result in a constant weight. The moisture adsorption of the carbon is expressed as follows:

Weight gain Dry weight A more detailed discussion of this method for determination of carbon moisture adsorption properties can be had when reference is made to Rubber World, March and April 1958, in which there appears a two-part article concerning this topic by E. M. Dannenberg and W. H. Opie, Jr. For reasons as yet not fully understood, carbons possessing the above-described combination of moisture adsorption and surface area requirements are unusually efficient reducing agents or catalysts for the reducible metal compounds. Thus, when the aforedescribed Moisture Adsorption Factor X types of carbons form part of the reducible metallurgical composition, the conversion or reduction step occurs both readily and rapidly.

Generally speaking, the particle size of the carbon utilized is not critical. It will be noted, however, that when finely-divided metal powders are to be produced, particulate carbons having an average primary particle diameter of less than about 100 microns and particularly less than about 1 micron are preferred.

Specific examples of readily available carbons which generally possess the above-described qualities are: car bon blacks chosen from the group consisting of oil fun nace blacks, channel blacks, acetylene blacks and gas furnace blacks, and various activated carbons.

The exact amount of the carbon to be admixed with any of the hereinbefore-described metal compounds will be determined primarily by the desired proportion of metal to carbon in the final metal composition. Also, the particular compound involved will determine the minimum amount of carbon to be combined therewith. For example, the minimum amount of carbon to be combined with the aforesaid metal compounds will normally bean amount approximately equivalent to the stoichiometric amount required to react with the metal compound to produce the desired amount of free metal. When organic metal compounds are involved (such as esters formed by reaction of organic acids with metallic bases) the minimum amount of carbon utilized will be somewhat less. However, in most cases the carbon utilized will not be less than about 1% by weight of the total mixture and preferably not less than about 2% by weight.

The process of the present invention is most valuable when applied to the production of finely-divided metal powder compositions of high purity, that is to say, metal powder compositions containing very small quantities of carbon, i.e. containing less than about 10% of carbon by weight of the total composition. Accordingly, in the most preferred embodiment of the invention, the amount of carbon utilized initially will rarely exceed the amount required to produce metal powder compositions COIl'lPI'lS- ing about 10% by weight carbon. In this respect however, it is to be understood that the amount of carbon in the metal composition can also be varied by post treatment of the composition to reduce the amount of residual carbon therein to any desired level. For example, the carbon content of the metal compositions produced in accordance with the invention can be adjusted by treatment with steam or in accordance with the teachings of commonly owned copending US. application Serial No. 521,542, filed January 19, 1966, by Dannenberg and Hardy.

The practice of our invention also includes the direct preparation of a metal powder essentially free from carbon residue without post treatment of the composition to reduce the amount of residual carbon. This aspect of our invention is best accomplished by combining an 1norganic compound of the aforementioned metals with an amount of carbon approximately equivalent to the stoichiometric amount required to reduce the metal compound to the elemental metal. oftentimes, it is advantageous to utilize an amount of carbon slightly less than the exact stoichiornetric amount required. After thermally converting such a mixture to produce the metal composition, the metal product is subsequently treated in a reducing atmosphere to convert any minor amounts of oxides present to the free metal. This procedure can insure an absolute minimum of residual carbon.

Our process can also be applied to the PIOdUCUOII Of finely-divided metal powder compositions comprising larger amounts of carbon. Such compositions can be utilized as fillers in elastomeric or plastomeric compositions and accordingly, can contain up to about 90% by weight of carbon if desired. Also, compositions containing varying amounts of carbon can be further treated under other than reducing conditions to produce other finely-divided metallurgical products in an essentially pure form or in combination with varying amounts of carbon. For example, the amount of carbon utilized in the reducible metal compositions can be initially selected in order to provide sufiicient carbon residue in the reduced metal composition so that treatment at more elevated temperatures in selected environments will convert the metal to the corresponding carbide or to a mixture of free metal and metal carbide. Obviously, this aspect of our invention is applicable only to those metal compounds mentioned above which will either react directly with carbon or, after reduction, will provide a free metal which will react with carbon to produce the carbide thereof. Included among such compounds are those of tungsten, molybdenum, chromium, vanadium and mixtures thereof.

In addition to the production of metals and metal powders containing predetermined amounts of carbon, the practice of our invention can also produce mixtures of metals and metal oxides. For instance, when nickel sulfate is reduced there normally occurs a two-stage reaction: (a) 'the nickel sulphate is decomposed to nickel oxide and (b) the nickel oxide is then reduced to the free metal. Obviously, by selecting an amount of carbon sufficient to react with only part of the nickel oxide, a mixture of nickel metal and nickel oxide will result upon thermal conversion under an inert atmosphere.

It is important that the reducible metal compound be in solution form; therefore, the choice of solvent will be dictated to a large extent by the solubility characteristics of said metal compound. For instance, it is obvious that when the preferred water soluble metal compounds are to be utilized, water will be the preferred solvent. However, it will also be recognized that certain reducible metal compounds are also soluble in solvent media other than water, for instance, in such organic media as alcohols, ethers, ketones, aldehydes, hydrocarbons, and the like. Thus, the use of such organic media is normally entirely suitable. Moreover, certain reducible metal compounds are substantially water insoluble but are readily soluble in certain organic media; hence, the use of such compounds in the process of the present invention will be restricted to the use of suitable organic liquid solvent media. For instance, ferric stearate, cobaltous linoleate, cobaltous naphthenate, ferrous oxalate, molybdenum hexacarbonyl and copper abietate are specific examples of reducible metal compounds which are substantially insoluble in water but which are readily soluble in certain organic media such as diethyl ether, benzene, methyl alcohol, acetone, and the like. It should be further noted that certain reducible metal compounds, although substantially insoluble in water, can be rendered sufficiently soluble when water is treated as by basidification or acidification. For instance, ferric succinate is normally insoluble in neutral water. Said compound is, however, soluble in aqueous acid solution. Likewise, cobaltous oxalate, although substantially insoluble in neutral water is nevertheless soluble in ammonia solution. Normally, therefore, suitable solvents for use in the present invention can be readily determined from examination of the solubility characteristics of the reducible metal compound to be utilized, bearing in mind, of course, that it is also obviously important that the liquid media utilized as the solvent be substantially inert with respect to the carbon and the reducible metal compound, i.e. that said liquid not react deleteriously with either the metal compound or the carbon.

The concentration of the metal compound in solution is generally not critical. Normally, it will be desirable for purposes of economy to utilize solutions of relatively high metal compound concentrations, i.e. comprising above about 5 weight percent of the metal compound, in order that as little solvent need be removed as possible in the subsequent drying step. However, it should be noted thatsolutions comprising as little as about 1 weight percent of the reducible metal compound are also entirely suitable.

A preferred general method for producing the reducible metal compound/carbon compositions of the present invention comprises slurrying the particulate carbon in the liquid medium, dissolving the reducible metal compound in said medium and thereafter removing the liquid medium prior to the thermal conversion step such as by drying. A particularly preferred method of removing said liquid medium resides in spray drying of such a slurry as taught by our hereinbefore mentioned U.S. application Serial No. 387,223. Obviously, it is generally of little importance whether, in the general process outline above, the metal compound is dissolved or the carbon slurried as the first step.

The temperature utilized to thermally convert the metal compound in the metal compound/carbon compositions to produce the free metal can vary over a wide range. In general, said range includes temperatures substantially below those normally required to reduce the metal compound as well as temperatures that can exceed said normal reduction temperature by 400 or 500 F. and even more. The lower temperatures are of special utility when the conversion is achieved by way of batch type processes. However, a more efiicient method of converting the metal compound to the corresponding metal is by way of a continuous process in which dry particles of the metal compound/carbon composition are conveyed through a high temperature conversion zone. In such continuous processes, it is obviously desirable to reduce residence time to a minimum and thus the temperature of the conversion zone will usually be relatively high.

The environment in the conversion zone will be determined by many factors such as the amount of carbon utilized, the conversion temperature utilized and the particular metal compound or mixtures thereof involved. For example, if the ultimate metal product is to be of high purity (i.e. low carbon content) then a reducing environment is often preferred. A reducing atmosphere is especially advantages when slightly less than the stoichiometric amount of carbon is initially mixed with the compound since said atmosphere insures essentially complete reduction and avoids further reduction steps. However, when larger amounts of carbon are involved, an inert atmosphere can also be utilized. An inert atmosphere is often particularly desirable when the metal product is to be a mixture of metal and metal oxide.

There follow a number of non-limiting illustrative examples:

Example 1 To an aqueous slurry consisting of about 320 grams H and 25 grams of Vulcan 9, an SAP oil furnace black produced by Cabot Corporation, having an average particle diameter of about 20 millimicrons, a surface area of about 125 m. /gram and a moisture adsorption factor of about 4%, there was charged a solution consisting of about 320- grams H 0 and about 100 grams The resulting mixture was mixed for about one hour and the water removed therefrom by heating in an oven at a temperature of about 250 F. for about 16 hours. The resulting cake was then ground in a Waring Blendor for about 4 minutes and the powdered product was sieved through a 325 Tyler mesh screen. About 5 grams of the screened product was then charged into each of four combustion boats. Next, said boats were placed in a nitrogen swept oven heated to about 1300 F. A crucible was withdrawn at the 5th, th, th, and 30th minute of heat treatment and cooled under a nitrogen atmosphere. The contents of each boat was then tested by X-ray diffraction analysis. Measurements of the nickel and nickel oxide peaks of the spectra were taken and are shown in Table I under the columns Ex. 1.

Example 2 This example is essentially a duplicate of Example 1 with the exception that the drying step was effected by spray drying. According, the aqueous nickel sulfate solution containing the carbon black was spray dried under the following conditons:

Rate cc./min 120 Inlet temp., F. 400 Outlet temp., F. 250

The dried product had an average particle size of about 44 microns. Said product was heat treated in accordance with the procedure of Example 1 and analyzed by X-ray diffraction (Table 1, columns Ex. 2).

Example 3 This example is essentially a duplicate of Example 1 with the exception that the carbon utilized was a Vulcan 9 carbon black which had been heat treated at about 2700 C. in an inert atmosphere. The BET N surface area was about 70 m /gram and the moisture adsorption factor about 0.05%. The average particle size of the black remained at about 20 millimicrons. The X-ray diffraction data is shown in Table I under the columns Ex. 3.

Example 4 While the ingredients and amounts utilized in the present example are essentially the same as utilized in Example 1, in the present example no solution of the nickel sulfate is effected. Accordingly, instead of utilizing water as the liquid solvent medium there is utilized acetone. Thus, to a slurry consisting of about 320 cc. of acetone and 25 grams of Vulcan 9 carbon black there is charged a slurry consisting of about 320 cc. acetone and about 100 grams of nickel sulfate hexahydrate which has been prilled and screened to about 325 Tyler mesh. The mixture is mixed for about one hour and the acetone removed therefrom by heating in nitrogen swept oven. Thereafter, the resulting metal compound/ carbon mixture is treated in the same manner as in Examples 1-3. Results of X-ray diffraction analyses of the heat treated product are shown in Table I under the columns Ex. 4. It is apparent that conversion to nickel metal of the metal compound/carbon composition provided in the present example occurs to a far lesser extent than the conversion of the metal compound/ carbon composition provided in Example 1.

TABLE I Ni NiO Ex. 2 Ex. 3 Ex. 4 Ex. 1 Ex. 2 Ex. 3 Ex. 4

Obviously, many changes can be made in the above examples and description without departing from the scope of the invention.

For instance, many metal compounds other than the nickel sulfate specifically utilized are also suitable such as cobalt acetate, ammonium paratungstate, nickel nitrate, zinc sulfate, and the like.

Likewise, carbons other than thermal and oil furnace blacks such as gas furnace, channel and acetylene blacks and activated charcoal are also suitable provided, of course, that said carbons adhere to the moisture adsorption and surface area limitations previously described.

What is claimed is:

1. In a process for producing metals which comprises heating to reducing temperatures a mixture comprising a reducible metal compound and carbon under non-oxidizing conditions, the improvement which comprises producing said mixture of reducible metal compound and carbon by (a) providing a solution comprising at least about 1 percent by weight of said reducible metal compound and an inert liquid medium solvent (b) mixing into said solution particulate carbon having a minimum surface area of about m. gram and a moisture adsorption factor of at least about 0.25%, and

(c) removing said solvent from the resulting mix ture.

2. The process of claim 1 wherein said carbon has a moisture adsorption factor of at least about 2% and a surface area of at least about 25 rn. /grarn.

3. The process of claim 1 wherein said carbon has an average particle diameter of less than about 1 micron.

4. The process of claim 3 wherein said carbon is chosen from the group consisting of Oil furnace blacks, gas furnace blacks, channel blacks and acetylene blacks.

5. The process of claim 1 wherein said metal compound is chosen from the group consisting of compounds of copper, tungsten, zinc, lead, tin, iron, cobalt nickel, manganese, vanadium, molybdenum, and mixtures thereof.

' 6. The process of claim 1 wherein said metal compound is water soluble and the solvent is water.

7. The process of claim 1 wherein said solvent is an organic liquid.

, References Cited UNITED STATES PATENTS 2,242,759 5/1941 Schlect et a1 7589 HYLAND BIZOT, Primary Examiner.

W. W. STALLARD, Assistant Examiner.

UNITED s ATEs PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,399,051 August 27, 1968 John F. Hardy et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 75, "mg/gram" should read 5 m. /gram Signed and sealed this 10th day of March 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Edward M. Fletcher, Ir.

Commissioner of Patents Attesting Officer 

